This invention relates to tag devices utilized in electronic article surveillance (EAS) systems and, more particularly, to EAS tag devices which can be deactivated and reactivated.
EAS systems are well known in the art and are widely used for inventory control and to prevent theft and similar unauthorized removal of articles from a controlled area. Typically, in such systems EAS tags are attached to the articles and a transmitter and an associated receiver are located at an exit to the controlled area. The transmitter serves to generate a field which interacts with any tags passing through the exit area or surveillance zone. The receiver serves to detect one or more predetermined detectable signals resulting from these interactions. Upon detection of a predetermined signal, the EAS system generates an alarm indicating the presence of a tag and its associated article in the surveillance zone.
One type of EAS tag which is in use today includes a nonlinear or mixing element, e.g., a semiconductor diode, and antenna elements. This tag is adapted to interact with the transmitted field by reradiating signals which can then be received by a system receiver,
An EAS system incorporating an EAS tag of the above-type is disclosed in U.S. Pat. No. 4,736,207, entitled TAG DEVICE AND METHOD FOR ELECTRONIC ARTICLE SURVEILLANCE, which issued to Silkaria, et al. on Apr. 5, 1988 and is assigned to the same assignee hereof. In the system of the '207 patent, a first high frequency electromagnetic field and a second lower frequency electrostatic field having a modulation characteristic are transmitted by the system transmitter. The EAS tag of the system interacts with these fields to cause the generation of an electromagnetic field which includes frequency components at the sum and difference of the high frequency and modulated lower frequency. These components are then detected by the system receiver to recover the modulation characteristic which results in generating the system alarm.
The '207 patent also mentions that the EAS tag can be deactivated. This is accomplished by providing access to the conductive tag members for applying a destructive energy pulse to one or more of the mixing diode or other circuit elements.
U.S. Pat. No. 5,257,009, in the name of Narlow and also assigned to the same assignee hereof, discloses an EAS tag of the '207 patent type in which the tag is made reactivatable and deactivatable by using a switching capacitor placed in circuit with the tag components. By using a remotely generated field, the capacitor can be switched between first and second values to deactivate and reactivate the tag. The '009 patent mentions that the capacitor can be placed in series or in parallel with the mixing diode. In the latter position, the capacitor provides different shunting of the signal to the diode at its different values, thereby promoting the desired deactivation and reactivation.
U.S. Pat. No. 4,063,229, entitled ARTICLE SURVEILLANCE, issued to Welsh, et al. and also assigned to the same assignee hereof, describes an earlier form of the tag of the '207 patent. The '229 patent describes a technique for deactivating the tag in which the tag is subjected to a remotely generated RF field to burn out the diode. The '229 patent also discloses a further deactivation technique in which the diode circuit is opened by a remotely generated field to deactivate the tag.
In this case, a whisker of soft magnetic material bridges the first and second leads of the diode which are connected to form a loop. By applying a DC magnetic field having a transverse flux to the tag, the whisker end attached to the positive lead of the diode is lifted from the lead to break contact and open the diode circuit.
Opening of the diode circuit in this manner, however, causes the tag to reradiate spurious signals to the receiver. Frequently, many of these signals are sufficient to cause the EAS system to undesirably generate a false alarm. Consequently, the tag is not completely deactivated.
It is, therefore, an object of the present invention to provide a tag that can be deactivated in a way which prevents reradiation by the tag of spurious signals in an EAS system.
It is a further object of the present invention to provide a tag meeting the above objective and which is remotely deactivatable and reactivatable as well as inexpensive to fabricate.